1. Field of the Invention
The invention relates to a charging protection method for a rechargeable battery and, more particularly, to a battery module and an overcharge protecting method.
2. Description of the Related Art
In a conventional portable electronic device, a high capacity lithium battery without memory effect is usually used as a built-in rechargeable battery. When the lithium battery is used in an electronic product, a plurality of lithium battery units are usually connected in series to provide a voltage to the electronic product. For example, in a notebook computer, three or four lithium battery units are connected in series to form a rechargeable lithium battery and provide an 11.1V or 14.8V battery voltage. However, since the voltage and electrical characteristic of each battery are different, the voltage and capacity of each battery unit are different. After several cycles of charge-discharge, the difference would become more obvious (which means the rechargeable battery gradually degrades).
The effect of the degradation of the rechargeable battery is more obvious when a quick charger is used to charge the battery. In detail, the lithium battery is taken as an example, when the lithium battery is continuously charged with a large current, the lithium battery unit which degrades more seriously is easy to be charged to a too high battery voltage (that is overcharge). When the overcharge occurs, the material in a lithium ion cell of the lithium battery decomposes, which results in the increase of the resistance and the temperature, and then combustible gas is generated. Consequently, if the overcharge frequently occurs, the lithium battery may overheat, burn or even explode.
In conventional technology, a common overcharge protection mechanism is launched when the rechargeable battery reaches a specific voltage. The charging process is stopped to avoid the overcharge. However, the overcharge protection mechanism takes effect and protects the rechargeable battery only after the overcharge occurs, so it cannot stop or avoid the overcharge. If the overcharge of the rechargeable battery occurs frequently, its degradation speeds up, and the service life and reliability of the rechargeable battery are affected seriously.